Detects whether or not TCP fastopen is supported by running sysctl to
grab the current value of net.inet.tcp.fastopen.server_enable.
If not enabled, directs the operator to use sysctl to set it temporarily
and /etc/sysctl.conf to set it on-boot.
Setting the socket option always works whether it is enabled or not.
Verified that fastopen works on FreeBSD 13. Did not attempt on earlier
versions but support has been there since FreeBSD 10.
Change-Id: I2e0c457558a6fa7b7a1b18bc3c6684aff50b81a2
